Axial centrifugal compressor with scalable rake angle

ABSTRACT

A mobile wheel for a centrifugal or axial centrifugal compressor for a turbine engine, that includes a base on which a plurality of vanes are attached at their feet and extend between a leading edge and a trailing edge, the trailing edge being inclined at the base of the vane by a slope angle relative to the middle plane extending through the base thereof in the rotation direction of the compressor. The trailing edge has at the head of the vane a slope angle smaller than the slope angle at the vane base.

The field of the present invention is that of turbomachines and moreparticularly that of turbomachine compressors comprising at least onecentrifugal or mixed (axial-centrifugal) compression stage.

Aeronautical turbine engines and in particular turbine engines forhelicopters generally comprise a compressor of the centrifugal or mixedtype as a compression stage of the gas generator. These compressors areformed by moving wheels or impellers which have a substantially axialair inlet and a more radial outlet, the blades being fixed on a base inthe form of a bell which serves as a guide for the air flow and divertsit from the inlet direction toward the outlet direction. The flow thuscirculates between this base and a compressor casing; it is compressedand propelled by the various blades of the impeller.

The head of the blading is free and positioned opposite the casing, orcover, which closes the air duct. The trailing edge is rectilinear inthe prior art and may be inclined with respect to the meridian plane ofthe impeller passing through the foot of the blade. The angle which itmakes with this meridian plane is designated by the term rake angle, orslope angle of the trailing edge, and constitutes one of the relevantgeometric parameters for describing a centrifugal impeller blading.

The choice of a significant slope angle (of the order, for example, of20°) at the trailing edge of a centrifugal impeller is a known means ofreducing the stresses at the blade foot which are due in particular tothe bending of the blading under centrifugal force. It thus allows ahigher-performance design of the compressor, and makes it possible inparticular:

-   -   to reduce, at equal speed of rotation, the thickness of the        blading, and hence that of the shell of the base, and thus to        significantly reduce the mass and the inertia of the rotor,    -   or else, at equal mass and inertia for the impeller, to increase        the speed of rotation.

Document U.S. Pat. No. 5,730,582, which describes a centrifugal impellerfor a turbomachine compressor, is known, for example. In this impeller,which has the shape of a helix inscribed in a cylinder, the slope angleis constant all along the trailing edge.

The consequence of choosing a significant slope angle, with arectilinear trailing edge, is that the head of the blading is inclinedby the same angle as the foot with respect to this meridian plane. Inthe case of accidental contact between the blading and the cover, if thetrailing edge is inclined in the direction of rotation of the impeller,the head of the blade has a tendency to auto-engage in the metal, whichrisks causing significant degradation.

Also known is document U.S. Pat. No. 4,819,884, which relates to aventilation device comprising an impeller in which the slope anglecontinuously increases from the foot of the blades toward their head andin which the head is held in a plate, thus avoiding any risk of theblades engaging in the casing of the device.

The aim of the present invention is to overcome these disadvantages byproviding a centrifugal or mixed axial-centrifugal compressor which doesnot have some of the disadvantages of the prior art and, in particular,which has a trailing edge with a significant slope angle at its foot,without there being produced a phenomenon of auto-engagement of the headof the blade in the case of contact thereof with the cover.

Accordingly, the subject of the invention is a moving wheel for acentrifugal or mixed axial-centrifugal compressor of a turbomachine,comprising a base on which a plurality of blades extending between aleading edge and a trailing edge are fixed by their feet, said trailingedge being inclined at the foot of the blade by a slope angle withrespect to the meridian plane which passes through it at its foot, inthe direction of rotation of said compressor, characterized in that thetrailing edge has at the head of the blade a slope angle β which is lessthan its slope angle α at the blade foot.

Since the slope angle at the head is smaller, the corner formed by thetrailing edge and the head of the blade moves more tangentially withrespect to the casing and is therefore less capable of penetratingtherein in the case of accidental contact.

Preferably, the slope angle β of the trailing edge at the blade head isclose to zero, or even zero. Even more preferably, the slope angle β ofthe trailing edge at the blade head is negative. These configurationsprovide greater assurance that the corner at the head of the trailingedge cannot become implanted in the casing in the case of accidentalcontact.

In one particular embodiment, the curvature of the trailing edge isconstant between the foot and the head of said trailing edge. Thetrailing edge thus has the shape of an arc of a circle.

In another embodiment, the curvature of the trailing edge iscontinuously variable between the foot and the head of said trailingedge. The trailing edge thus has the shape of a helix.

The invention also relates to a turbomachine comprising a compressorhaving one of the characteristics described above. Finally, it relatesto an aircraft equipped with such a turbomachine.

The invention will be better understood, and other aims, details,characteristics and advantages thereof will emerge more clearly, in thecourse of the following detailed explanatory description of anembodiment of the invention given by way of purely illustrative andnonlimiting example, with reference to the appended schematic drawings,in which:

FIG. 1 is a perspective view of a centrifugal compressor impelleraccording to an embodiment of the invention;

FIG. 2 is a front view of a trailing edge of a centrifugal compressorblade according to the prior art, and

FIG. 3 is a front view of a trailing edge of a centrifugal compressorblade according to an embodiment of the invention.

With reference to FIG. 1, there can be seen a centrifugal compressor 1comprising a base 2, in the form of a bell, on which compressor blades 3distributed uniformly over the periphery of the base 2 are fixed. Thebase 2 terminates at its upper part, which corresponds to the inletplane of the air to be compressed, in a cylinder forming a hub 4 for thecompressor, and, at its lower part, in a disk 5 oriented radially withrespect to the axis of rotation of said compressor. The blades 3 areheld by the base 2 at their feet 6; the heads 7 of these blades are freeand rotate inside a compressor casing (not shown). In the direction offlow of the air stream, the blades extend between a leading edge 8situated at the hub 4 and a trailing edge 9 situated at the disk 5.

With reference now to FIG. 2, there can be seen a trailing edge 9 of acompressor blade 3, according to the prior art. This trailing edge is,at its foot, inclined with respect to the meridian plane which passesthrough it, in the direction of rotation, by a slope angle α. Thistrailing edge 9 is rectilinear, that is to say that it has the sameinclination, equal to its value α at the blade foot, over all itsheight, from the foot to the head of the trailing edge 9.

By contrast, in FIG. 3, the trailing edge 9 has a shape which ischangeable along its height. It is inclined by a slope angle α in thedirection of rotation, at the foot 6 of the blade 3, which is defined asin the prior art, but this angle becomes progressively smaller in theupward direction along the trailing edge so as to reach a value β at thehead 7 of the blade 3, which is less than α. This value β may be zero oreven negative, that is to say that the head of the blade is theninclined in the opposite direction to the direction of rotation of theimpeller.

The proposed invention therefore consists in choosing, at the trailingedge 9, a large slope angle α at the blade foot 6, then in applying acurvature to the blading 3 so as to find a smaller, or even zero ornegative, slope angle at the head of the blading. The presence of alarge slope angle α at the blade foot 6 makes it possible to reduce thebending stresses at the foot, whereas the small slope angle β at thehead 7 reduces the risks of auto-engagement of the blade 3 in the casingin the case of accidental contact between the head 7 of the blade at thetrailing edge 9 and the casing.

The production of such a profile at the trailing edge 9 is based on thepoint machining method which allows 3 D blading geometries; by contrast,the flank machining previously used restricted the possibilities byimposing ruled surfaces (that is to say consisting of a stack ofstraight lines) for the design of the profile of the blades.

This modification of the geometry of the profile is not manifested by asignificant aerodynamic impact. The use of blades 3 according to theinvention even contributes, in a surprising manner, rather to anincrease in the performance of the compressor, resulting from thereduction in the slope angle along the height of the trailing edge 9.

The dual reduction of the stresses in the blading and of theauto-engaging nature of the head of the blade, by application of theinvention, have been verified by calculation on actual blading.

The invention therefore allows a saving in mass and inertia or, withequal mass and equal inertia, an increase in the speed of rotationwithout degrading the behavior in the case of contact between the blade3 and the casing.

Many variants can be envisioned for the change in the curvature of theblade 3 at the trailing edge 9 between its foot 6 and its head 7. Thecurvature may, for example, be constant, the trailing edge having theshape of an arc of a circle, or else be continuously variable, theprofile of the trailing edge then having the shape of a helix. Thecurvature between the foot and the head of the blade is chosen accordingto the impact that it has on the aerodynamic performance of thecompressor.

Although the invention has been described in relation to a particularembodiment, it is clearly obvious that it is not in any way restrictedthereto and that it encompasses all the technical equivalents of themeans described and combinations thereof where these fall within thescope of the invention.

1-8. (canceled)
 9. A moving wheel for a centrifugal or mixedaxial-centrifugal compressor of a turbomachine, comprising: a base onwhich a plurality of blades extending between a leading edge and atrailing edge are fixed by their feet, the trailing edge being inclinedat a foot of the blade by a slope angle α with respect to the meridianplane that passes through it at its foot, in a direction of rotation ofthe compressor, wherein the trailing edge has at the head of the blade aslope angle β that is less than its slope angle α at the blade foot. 10.The moving wheel as claimed in claim 9, in which the slope angle β ofthe trailing edge at the blade head is close to zero, or is zero. 11.The moving wheel as claimed in claim 9, in which the slope angle β ofthe trailing edge at the blade head is negative.
 12. The moving wheel asclaimed in claim 9, in which curvature of the trailing edge is constantbetween the foot and the head of the trailing edge.
 13. The moving wheelas claimed in claim 9, in which curvature of the trailing edge iscontinuously variable between the foot and the head of the trailingedge.
 14. A compressor comprising a moving wheel as claimed in claim 9.15. A turbomachine comprising a compressor as claimed in claim
 14. 16.An aircraft comprising a turbomachine as claimed in claim 15.